The objective of the 3GPP Long Term Evolution (LTE) is to provide enhanced performance in terms of higher data rates, reduced delays, improved coverage, e.g. the percentage of a network service area that upholds a required communication quality, and capacity, e.g. the maximum amount of data that can be transmitted over a channel. The downlink air interface is Orthogonal Frequency-Division Multiple Access (OFDMA) based, while Single Carrier FDMA (SC-FDMA) will be employed in the uplink. The downlink scheduler and uplink scheduler at the base station side performs the resource allocation for users.
When data is available at the transmit buffers of a user equipment (UE), and the user equipment has no grant for transmission on a Physical Uplink Shared Channel (PUSCH), the user equipment need to request permission to transmit. A Scheduling Request (SR) will either be transmitted on the Random Access Channel (RACH), denoted as RA-SR, or on dedicated resources on the Physical Uplink Control Channel (PUCCH), denoted as D-SR. The Uplink (UL) scheduler monitors the users' requests and distributes the available resources among the users.
At the user equipment side, there are multiple priority queues for different services. Each priority queue has a specific priority, and is valid for one service. Commonly, Radio Resource Control (RRC) signaling may be served with highest priority both in uplink and downlink. Measurement results of received signal power for both a source cell serving the user equipment and also neighbouring cells are reported from the user equipment to the radio base station side periodically or when a handover event is triggered. When a user equipment moves to the cell edge and the radio channel quality of the source cell is several dB lower than the radio channel quality from the strongest neighbouring cell for a time-to-trigger interval, a Handover (HO) event will be triggered. The radio base station will make a handover decision and send a handover command to the user equipment. The user equipment then tries to reach the target cell and do a handover. When the handover procedure is ready, the user equipment responds to the radio base station with a handover confirm message.
Handover related signaling has strict delay requirements, such as for example RRC signaling. If one of the handover messages is transmitted with too long delay, the handover may fail. This failure may cause the user equipment to remain connected to a non-optimal cell, and may have a great negative effect on the service performance perceived by the user.